1. Field of the Invention
The present invention relates generally to a switching type power supply circuit suitable for use with a power supply circuit of a television receiver, and is directed more particularly to a switching type power supply circuit which will restrict the output current upon an over load of its switching element, such as might occur if the load of the power supply circuit were short-circuited, and also prevent the input voltage of the power supply circuit from being applied directly to its load circuit if its a switching element is short-circuited.
2. Description of the Prior Art
In the art two basic types of switching power supply circuits are known the parallel type switching power supply circuit, in which the primary winding of a transformer and a switching element are connected in series across a DC input voltage and in which a rectifying circuit is connected to the secondary winding of the transformer, and the series type switching power supply circuit, in which a switching element is connected in series with a choke coil across a DC voltage supply. An advantage of the parallel type power supply circuit, such an advantage is obtained that an output voltage higher than the input voltage can be generated in accordance with the winding number ratio of the transformer, but such a defect is accompanied that its efficiency is low and it requires the transformer.
An example of the prior art series type power supply circuit will be now described with reference to FIG. 1. This example consists of a diode bridge 2 for rectification which is connected to an AC power source 1, a smoothing capacitor 3 which is connected between output terminals 2a and 2b of the diode bridge 2, a transistor 4 which is supplied at its collector with the rectified output from the diode bridge 2 through the capacitor 3, a choke coil 6 which is inserted between the emitter of the transistor 4 and an output terminal 5, a fly-wheel (current holding) diode 7 connected between one end of the choke coil 6 and the ground, and a capacitor 8 connected between the other end of the choke coil 6 and the ground. In the prior art circuit of FIG. 1, the transistor 4 achieves the switching operation in response to the duty factor of a switching pulse applied thereto through a drive transformer 9, and the input voltage is reduced by the ratio in accordance with the duty factor to produce an output voltage which is applied to a load circuit 10 connected to the output terminal 5.
The above prior art series type power supply circuit has such an advantage that it is good in efficiency as compared with the parallel type power supply circuit and the choke coil 6 of a small size and hence an inexpensive one is sufficient, but can not be free from such a disadvantage that when the transistor 4 is damaged and its collector-emitter path is short-circuited, the input voltage is, as it is, applied to the load circuit 10, which is very dangerous.
If the above is taken into account, such a power supply circuit as shown in FIG. 2 may be considered in which reference numerals same as those used in FIG. 1 designate the same elements. In case of FIG. 2, the output terminal 2b of the diode bridge 2, which connected to the collector of the transistor 4, that is, output terminal 2b, is not grounded, but instead is connected to the output terminal 5.
With the circuit of FIG. 2, during the time period when the transistor 4 is ON, current flows through the transistor 4 and choke coil 6 and energy is stored in the choke coil 6. Thus, during the time period when the transistor is OFF, current flows through fly-wheel diode 7 and the choke coil 6 to capacitor 8 the energy previously stored in the choke coil 6. Accordingly, the capacitor 8 is charged up and power is applied to the load circuit 10. The output voltage appearing at the output terminal 5 is a voltage provided by reducing the input voltage according to the duty factor of the switching pulse applied to the transistor 4. In the event that the transistor 4 is damaged and its collector-emitter path is short-circuited, both ends of the smoothing capacitor 3 are short-circuited, and hence the input voltage is not applied to the load circuit 10. If a fuse is inserted between the AC power source 1 and the diode bridge 2 in FIG. 2, the fuse would melt as a result of such a short circuit and protect the load circuit.